1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to air-fuel control upon a change in the operating condition of the engine, i.e., air-fuel ratio control applied when the engine is in a transient condition.
2. Prior Art
Conventionally, an air-fuel ratio control system for internal combustion engines is well known, which feedback-controls the air-fuel ratio of a mixture supplied to the engine to a desired value in response to the output from an exhaust gas component concentration sensor arranged in the exhaust system and having an output characteristic in approximate proportion to the concentration of an exhaust gas component.
In an air-fuel control system of this kind, an air-fuel ratio correction coefficient is calculated in response to the output from the exhaust gas component concentration sensor, and the air-fuel ratio of the mixture is feedback-controlled to a desired value by the use of the air-fuel ratio correction coefficient.
The above-mentioned air-fuel ratio correction coefficient is calculated by adding together a P-term KLAFP, an I-term KLAFI, and a D-term KLAFD obtained, respectively, by proportional control (P-term control), integral control (I-term control), and differential control (D-term control) which are executed in response to operating conditions of the engine.
In the air-fuel ratio control system, since the air-fuel ratio correction coefficient is calculated in response to operating conditions of the engine as mentioned above, the air-fuel ratio correction coefficient varies depending upon a change in the operating mode which is determined by operating conditions of the engine, etc.
However, when the air-fuel ratio flactuates due to a change in the operating mode, there is a time lag between the time of occurrence of fluctuation and the time the exhaust gas component concentration sensor detects the flactuation. As a result, if the air-fuel ratio is feedback-controlled by the use of the air-fuel ratio correction coefficient which is changed at a change rate appropriate to a steady operating condition of the engine, upon a change in the operating mode, the air-fuel ratio largely deviates from the desired value, which results in degraded exhaust emission characteristics due to an increase in the amount of noxious components in the exhaust gases, as well as in degraded driveability.